The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a smartphone and a cellular phone, a digital camera, an infrared camera, a digital video camera, an onboard camera, a network camera, a TV conference camera, a fiberscope, and a capsule endoscope.
Some vehicles are equipped with a plurality of cameras for improving safety and/or convenience. For example, as a camera to support the driver to safely back a vehicle, there is a backup camera. Since the rear view from a driver's seat is poor, even if the driver is extra careful, the vehicle may hit an obstacle during backing, which may result in an accident. In case of a vehicle equipped with a backup camera, the view behind the vehicle is displayed on a monitor upon backing the vehicle. Since the driver can visually check an object(s) behind the vehicle even when the object is hard to see due to shadow behind the vehicle, the driver can safely back the vehicle without hitting an obstacle. Such camera for mounting in a vehicle, so-called “onboard camera”, is expected to be continuously more demanded.
Such onboard cameras may be mounted in relatively small spaces, such as a back door, a front grille, side mirror, and interior space thereof. For this reason, an imaging lens for mounting in an onboard camera is required to have a compact size. Further, the imaging lens for an onboard camera is required to attain high resolution suitable for high pixel count of an imaging element and a wide angle of view to take a wider range of an image. However, when downsizing of an imaging lens is attempted, refractive power of each lens becomes strong, so that it is difficult to satisfactorily correct aberrations. Therefore, it is difficult to attain a wider angle of view, in addition to satisfying the requirements such as small size and high resolution, while satisfactorily correcting aberrations. Upon actual designing of an imaging lens, it is a key to meet those requirements in a balanced manner.
On the other hand, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, i.e., multifunctional cellular phones which can run various application software as well as a voice call function, have been more widely used. The product lineup of smartphones is very wide including beginner models to high-end models, and is often categorized by performance of hardware, optical performance of a camera, etc. Among those high-end models, there is a model that is intended to have a new added value by equipping two imaging lenses. For example, in case of a conventional model having an imaging lens having a wide angle of view as well as an imaging lens of typical angle of view, images from those two imaging lenses are synthesized being processed by software, so as to achieve smooth zooming in and zooming out. Similarly to the imaging lense of the onboard camera, the imaging lens used for such purpose is required to have even smaller size, as well as high resolution and wider angle of view.
For example, as such conventional imaging lens having a wide angle of view, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2016-29501
According to the Patent Reference, the conventional imaging lens includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens, arranged in the order from the object side. The first lens is negative and has a meniscus shape directing a convex surface thereof to an object side. The second lens is negative, the third lens is positive, the fourth lens is positive, and the fifth lens is negative. According to the conventional imaging lens, it is achievable to attain small size and a wide angle of view of the imaging lens, by satisfying a plurality of conditional expressions for distances on an optical axis between lens surfaces, a conditional expression for a distance from an object-side surface of the first lens to an image plane and a focal length of the whole lens system, a conditional expression for a focal length of the third lens and a focal length of the whole lens system, a plurality of conditional expressions for curvature radii of the fourth lens and the fifth lens, a conditional expression for a back focal length and a focal length of the whole lens system, etc.
In these years, there are more diverse demands for an imaging lens of a wide angle of view. Especially in these years, there is higher demands for cameras that enable users to take images of objects even in the dark. Therefore, even in case of an imaging lens for mounting in a camera, it is necessary to ensure the imaging lens to have satisfactory optical performance in the dark. In order to take an image of an object in the dark, for example, it is necessary to irradiate near-infrared light to the object from the camera and take picture of the reflected light. However, the wavelengths of near-infrared light beams are longer than those of visible light beams. Therefore, in case of typical wide-angle lenses, a focal position of a near-infrared light significantly shifts relative to that of a visible light and it is difficult to form an image of the object on an imaging element. Therefore, in case of an imaging lens of this type, it is necessary to have an image-forming performance in the range of near-infrared light as well as the range of a visible light.
According to the conventional imaging lens of Patent Reference, although the number of lenses that composes the conventional imaging lens is as small as five, the imaging angle of view is wide, and aberrations are relatively satisfactorily corrected. However, in case of conventional imaging lens having wide angles, such as the one described in Patent Reference, it is difficult to obtain satisfactorily image-forming performance over wide wavelength range from that of visible light to that of near-infrared light. As one of solutions for such problem, an optical element may be inserted/removed between an imaging lens and an imaging element so as to adjust a focal length. However, to do so, the imaging lens or the camera has to include a mechanism for inserting/removing the optical element, which is not preferred in view of downsizing of the imaging lens and/or the camera.
Here, such problem is not unique to the imaging lens for mounting in an onboard camera and smartphones. In case of security cameras, it becomes a “must-have” function to be able to take an image by infrared radiation so as to monitor after sunset. In case of digital cameras and digital video cameras, there are already products available in the market, which are equipped with a night vision function. In addition, in case of network cameras, TV conference cameras, and cameras of fiberscopes and capsule endoscopes, there are products available, which have a function of taking images in a near-infrared range. The above-described problems are common among imaging lenses for mounting in relatively small-sized cameras of this type.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain a wider angle of view and satisfactory correct aberrations, while achieving a small size.
Further objects and advantages of the present invention will be apparent from the following description of the invention.